1. Field of the Invention
This invention relates to a generator for generating digital video signals suitable for use for the measurement or adjustment of a digital video appliance such as, for example, a video tape recorder of the digital recording method, and more particularly to a digital video signal generator for generating digital video signals, for which a great memory capacity is required for every kind of signals in a broadcasting system or in a high quality television system wherein the period for which the phases of a horizontal synchronizing signal and a subcarrier are to be synchronized correctly with each other is long, with a hardware construction of a small memory capacity and a small scale.
2. Description of the Prior Art
A video signal generator is employed for the measurement, adjustment, inspection and so forth of a video tape recorder (VTR).
A conventional analog signal generator for an analog VTR can generate normally to 10 to 20 kinds of signals, or in some cases, 30 or more kinds of signals. However, video signals handled in a VTR of the digitally recording type are digital signals of, for example, 8 to 10 bits/sample, and a conventional analog video signal generator cannot be used for such VTR of the digital recording type. Simply, it may be an idea to convert an output of an analog video signal generator into a digital signal. However, a digital signal obtained by such analog to digital conversion does not have a sufficient degree of accuracy for the measurement or adjustment of a digital VTR. Thus, it is a conventional method to write desired signals in advance into read only memories (ROMs) for individual kinds of signals as seen in FIG. 4 and designate an address of one of the ROMs in which a necessary signal is stored to read out the necessary signal. However, such conventional method requires a great ROM capacity, and there is a problem that, each time a kind of a necessary signal increases, a ROM for the storage of such necessary signal is additionally required. The problem will be described by way of an example of a video signal.
Referring to FIG. 3(a), there is illustrated an example of a screen when a waveform can be produced freely in a horizontal direction of the screen but it is only necessary for an indication on the screen to have same patterns in a vertical direction. The screen shown includes an indication of full field color bars.
FIG. 3(b) illustrates an example of a signal of the n-th line of the screen shown in FIG. 3(a). Here, a phase relationship between a horizontal synchronizing signal (H-SYNC) and a color subcarrier (particularly a burst signal) will be examined and a memory capacity necessary for the storage of digital video signal will be calculated. It is to be noted that, since a memory capacity for signals for a vertical synchronizing signal interval can be assured with a small hardware construction, such memory capacity is not taken into consideration here.
First, in the case of the NTSC system, if the sampling frequency f.sub.s is selected to four times the subcarrier frequency f.sub.sc, since 4.times.f.sub.sc =f.sub.s =910 f.sub.H stands where f.sub.H is a frequency of a horizontal synchronizing signal, 2.times.f.sub.sc =455.times.f.sub.H is obtained by conversion, and it can be seen that the burst signal is divisible for each 2 lines.
Such relationship is illustrated in FIG. 3(c) wherein the burst signal is reversed in phase for every other line so that the original phase is restored for each two lines. While the n-th line and the n+1-th line look in the same colors on a monitor screen, when a signal is to be produced using a ROM, a memory capacity for two lines is required.
Accordingly, a memory capacity necessary for a single signal for a screen which includes an indication of same vertical patterns includes 910 (samples).times.10 (bits).times.2 (lines)=18,200 bits, and if a condition is provided that a screen has an indication of same vertical patterns, then the capacity of a ROM for the NTSC system can be decreased.
However, the conditions are different in the case of the PAL system. If also the sampling frequency f.sub.s =4f.sub.sc is selected, then since there is a relationship of 4.times.f.sub.sc =f.sub.sc =(1,135+4/625)f.sub.H, 4.times.625.times.f.sub.sc =(1,135.times.625+4)f.sub.H is obtained. Thus, 4.times.625 lines=4 frames=8 fields are required to restore an original phase relationship between a horizontal synchronizing signal and a subcarrier.
Accordingly, a memory capacity necessary for signals of the PAL system is (1,135.times.625+4) (samples/frame).times.4 (frames).times.10 (bits).div.28.4 Mbits, and if the number of kinds of signals is 10, then a total of 284 Mbits are required, and if the number of kinds of signals is 30, then a total of 852 Mbits are required. Such large memory capacity requires a large number of devices even if recent large capacity ROMs are employed. Accordingly, such a small-sized, light-weight, inexpensive digital video signal generator as can be carried by a service man, for example, for the repair of a digital VTR, cannot be anticipated.
A similar problem takes place also with a digital video signal generator such as a digital video signal generator of the high quality television system where a large memory capacity is required for one kind of a signal.